The present invention relates to an FeRAM memory chip, as well as to a method of operating such a memory chip.
FeRAM memory chips are now in common use. One beneficial characteristic of these chips is that they are non-volatile, which means that if the power supply to the FeRAM memory chip is removed, the data stored in the device is not deleted. This characteristic is in contrast to certain other memory devices, such as DRAM devices, which forget their data as soon as the power supply is switched off.
A typical application for a non-volatile memory chip is a RAM memory for a low power device. Such devices may have an unreliable power supply, or may be designed to stop drawing power whenever possible, such as when the device is not in use. The advantage of providing a low-power device with a non-volatile memory chip as the RAM memory is that, after a power down, any code (e.g. operating system program code or application programs) stored in the non-volatile memory is not deleted. This means that then the device is powered-up again, the system would be in the same state as before the power off. The device does not have to be rebooted for example.
Another application of non-volatile memory chips is as simple data storage devices.
The overall structure of a known FeRAM memory chip is shown schematically in FIG. 1. The device has a number of pins including pins which are referred to in this document as data transfer pins. The data transfer pins are for receiving data to be stored and address data indicating where in the memory the data should be stored, and for outputting data stored in the memory cells. In some memory chips the pins for receiving the data and addresses (xe2x80x9cinput pinsxe2x80x9d) are different pins from the pins which output data (xe2x80x9coutput pinsxe2x80x9d). However, in other memory chips the same pins may act as input or as output pins at different times.
The memory chip shown in FIG. 1 includes two input pins marked as xe2x80x9cdataxe2x80x9d and xe2x80x9caddressxe2x80x9d. The pins xe2x80x9cdataxe2x80x9d and xe2x80x9caddressxe2x80x9d respectively receive data to be stored in the device and an address at which that data is to be stored. The memory chip further includes control pins (not shown), such as a chip enable (CE) pin, or an output enable (OE) pin, for receiving commands.
The memory chip has an address decoder which includes a word line address decoder (not shown) which uses the address to obtain a word line address (corresponding to a row of the memory array), and a column decoder 1 which uses the address to obtain the column address.
The memory chip further includes for each column a corresponding sense amplifier 3. Only one of these is shown in FIG. 1 for simplicity. The column address selects a sense amplifier 3, and the word line address (row) selects one cell of the corresponding column. The sense amplifier 3 is arranged to read data out of the selected cell of the selected column, or to write data to into the selected cell of the selected column, based on the a clock signal received through a clock pin (not shown) of the memory chip and the control signals (e.g. CE, OE).
As shown in FIG. 1, the columns are each made up of two chain cell arrays 5, and the sense amplifiers 3 are shared between those two adjacent chain cell arrays 5. For example, word lines addresses 0, . . . 255 might be in the left hand chain cell array 5, and word line addresses 256-511 might be in the right hand chain cell array 5. However, many other possible arrangements are known.
The present inventors have appreciated that, although non-volatile FeRAM memory devices are certainly useful, there are circumstances in which it is not desirable that the data stored inside the memory should be retained.
For example, following a system failure, it would be helpful if all system components could be reset into a defined state, rather than simply returning to their state before the system failure. Furthermore, if a low power device experiences an application failure such as a crash, it would be desirable if the data in the memory (which may have led to the crash) is removed, and if the application program itself is erased from the memory. Furthermore, in cases in which the memory chip is used for storage only, a deliberate erasal of the memory may be desirable when the device is used in an environment with only limited operating system capabilities.
For this reason, the present invention proposes in general terms that a FeRAM memory chip should include a reset unit for recognizing an externally applied reset signal. The reset unit is arranged upon recognition of this reset signal to initiate an operation in which at least a portion, and preferably all, of the FeRAM memory is erased.
The reset unit may be arranged to perform this reset operation as a sequence of reset steps in which respective sections of the memory are reset. In this way, the peak power required to perform the reset operation may be kept to an acceptably low level. A balance can be achieved between the speed of the reset and the power requirement.
There are various forms which the reset signal may take in different embodiments of the invention.
One option is for the memory chip to have a dedicated pin input for receiving a reset signal, so that the reset unit can initiate the reset operation upon measuring that a voltage on this pin reaches a certain level.
Another option, which does not require a dedicated pin and therefore which is cheaper to implement, is for the reset unit to be sensitive to a reset signal applied as voltages on one or more pins which are otherwise used for inputting data to the memory chip.
For example, the reset signal may be that the voltage on one of the input pins is set to a voltage which is not encountered in the usual operation of the chip (e.g. a voltage which is higher than the voltages which are encountered on that input pin when data is transmitted to the memory chip for storage).
Alternatively, the reset signal may be a pattern of voltages over a period of time which is not encountered in the usual operation of the memory chip, such as a non-standard sequence of voltages on the clock-pin.
More generally, the reset signal may be any xe2x80x9csoft entryxe2x80x9d command. xe2x80x9cSoft entryxe2x80x9d is a sequence of non-standard clock signals and commands (e.g. a series of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d s on selected data pins) applied to the memory chip""s data pins. As in some known memory chips, the chip according to the present invention may have a soft entry decoder unit for recognising a limited set of commands (e.g. that the voltage power input VDC is the default VDC plus 30 mV) as a reset signal.
A further aspect of the invention provides a method of using a memory chip as defined above, comprising supplying a reset signal to the memory chip, whereby the reset unit instates an operation in which at least a portion, and preferably all, of the FeRAM memory is erased.
A further expression of the invention is a device, such as a low power device, comprising a memory chip as defined above, and a processor operative to store data in the memory chip and retrieve the stored data from the memory chip, the processor further being operative to generate a reset signal and transmit it to the memory chip.